Semiconductor devices having silicon carbide as base material are continuously developed to be used in connection with high temperatures, high power applications and under high radiation conditions. Under such circumstances conventional semiconductors do not work satisfactorily. Evaluations indicate that SiC semiconductors of power MISFET-type and diode rectifiers based on SiC would be able to operate over a greater voltage and temperature interval, e.g. up to 650-800.degree. C., and show better switching properties such as lower losses and higher working frequencies and nevertheless have a volume 20 times smaller than corresponding silicon devices. These possible improvements are based on the favorable material properties that silicon carbide possesses in relation to silicon, such, e.g., a higher breakdown field (up to 10 times higher than silicon), a higher thermal conductivity (more than 3 times higher than silicon) and a higher energy band gap (2.86 eV for 6H-SiC, one of the crystal structures of SiC).
SiC semiconductor technology is relatively new and in many aspects immature. There are many critical manufacturing problems that are to be solved before SiC semiconductor devices may be realized experimentally and large scale manufacturing may become a reality. This is especially true of devices intended for use in high-power and high-voltage applications. SiC device manufacturing usually has an SiC substrate as starting material. For SiC devices to be commercially interesting for large scale production the substrates have to be produced at a low cost. Substrates are usually cut from a single crystal boule. There are a few alternative methods to grow a single crystal boule as described by V. F. Tsvetkov et al. in "Recent progress in SiC crystal growth", Inst. Phys. Conf. Ser. No 142: Chapter 1, but the only technique for large scale production of SiC substrates that has shown promise to date is seeded sublimation growth. However, one drawback of this technique is that the boules, and thus also the substrates contain hollow penetrating defects, which are usually referred to as micropipes. These defects are caused by a number of mechanisms and are in fact small diameter holes, which may extend all the way through the boule in the growth direction. Typically, the diameter of the holes is 0.1-5 .mu.m. The micropipes are inherent to further layers grown epitaxially on a substrate containing micropipes. The micropipes are therefore harmful for high voltage devices, as was initially described by P. G. Neudeck et al., "Performance limiting micropipe defects in silicon carbide wafers", IEEE Electron Device Lett. 15, 63 (1994). Apart from the hollow, seeded sublimation grown SiC boules also exhibit a number of other types of defects, such as dislocations and stacking faults.
High power devices are normally designed as vertical devices with the current transport mainly perpendicular to the substrate surface in order to get a large area through which the on-state current passes. A given design current gives a minimum area for the current transport. The micropipes cannot be present within the device as they will greatly reduce the maximum reverse voltage over the device. Thus, a large area free from micropipes is required to make the substrate suitable for a device capable of handling high currents. State of the art substrates have a micropipe density of between 1 and 200 per cm.sup.2, limiting the maximum area available for a device. Components that require a higher current than is feasible with a single SiC device have to rely on a number of devices arranged parallel to each other. However, as the single devices are not identical with each other care has to be taken when designing the component to avoid breakdown of a single device within the component during operation. Furthermore, increasing the number of devices within a component makes packaging a challenge.
There are a few other types of crystal defects apart from micropipes, e.g. dislocations and stacking faults. However, these types of defects have not shown to significantly effect the function of a high voltage device. Micropipes can in principle be found on SiC substrates by inspection in a phase-contrast optical microscope "T. Kato, M. Ohato, M. Razeghi and T. Okudo, IOP Conf. Proc. 142, pp. 417-420 (1996)". Other techniques that are used for detecting the micropipes include x-ray topography.
In the Japanese patent application 7-175045 a method is disclosed of how to reduce the leakage current in a pn junction. The method is based on heat-oxidizing the inner wall of the micropipe. However, the method does not eliminate the problem for high power devices designed to block voltages close to the theoretical limit of SiC. Furthermore, if a hole is still present after the oxidation of the inner walls of the micropipe, this will decrease the maximum reverse voltage the device can block.